Traditional coverlays currently used in the electronic industry are typically only available as polyimide-type dry films or foils and are supplied as sheets or rolls. The use of these materials involves high cost in terms of manpower, material consumption (including wastage) and energy time expenditure. These dry film sheets or rolls can only be applied using a time and cost intensive processes, especially if they are to be used only on selected areas of a panel. To apply coverlays in the form of dry sheets or rolls to the selected areas would require multiple process steps. For example:                1. Cutting those areas from a sheet or a roll by use of laser cutter or cut plotter.        2. Manually removing the protective layer from the adhesive side—the foil is normally supplied with epoxy or acrylic adhesive. Adhesiveless polyimide foils need a separate adhesive which is again supplied in rolls and which needs to be prepared the same way as the polyimide foil.        3. Positioning the polyimide foil (and the adhesive if separate) onto the panel in manual operation with best possible accuracy (>0.8 mm).        4. Fixing the polyimide foil in manual process using a soldering tool.While it is possible to carry out steps 1-4 by use of automation (e.g. robots) saving significant manual labor and associated expenses, it can be appreciated that the cost of investing in and operating the multiple robots necessary to carry out these operations sequentially is extremely high.        
Dry film polyimide coverlays also have technical problems when used in the manufacture of electronic components, such as printed circuit boards, mainly due to the need to use adhesives in combination with the polyimides that can lead to humidity absorption, smearing, dust and limited accuracy of positioning which does not meet the accuracy requirements of modern technology.
Other associated problems with using dry film polyimide coverlay films for mechanical processes such as flexible printed circuits (FPCs) as well as other printed circuit boards (PCBs) and other electronic components include:                Limited accuracy (0.8 mm minimum) due to manual positioning of the films.        Dimensional problems across the z axis.        Foil instability due to manual mounting and press processes.        Compatible only with selective metallization processes. Acrylic adhesive bleeds therefore plasma desmear* required as an additional process step.        Technical problems arise with machining, drilling and routing which include smear and dust.        Each process cycle is very time consuming. *A general description of “smear” and “desmear” can be found in the following reference: A Comprehensive Guide to the Design and Manufacture of Printed Circuit Board Assemblies—Volume 2, William Macleod Ross, Electrochemical Publications, page 232.        
Flexible printed circuit (FPCs) constitute an increasingly strong growth area in printed circuit board manufacturing, as they offer numerous advantages over rigid circuit panels. FPC's offer electronic equipment manufacturers the advantage of flexibility and compact high density wiring with high reliability, weight reduction and an overall cost saving. FPC's have been in use since the late 50's/early 60's mainly in military/space applications, however more recently they are more commonly found in retail products such as cameras, mobile phones and MP3 players.
The current manufacturing techniques for producing FPC's are described in Printed Circuit Board Materials Handbook by Martin W. Jawitz, McGraw-Hill Professional, 1997, page 784. This highly detailed book explains FPC construction and describes/discusses the assembly steps involved.
One important FPC component is a covering layer (also known as a cover sheet or coverlay); this is a plastic film on the top surface of the FPC to protect it from electrical crossovers and mechanical handling. In traditional flexible printed circuit board fabrication used to produce multi-layer circuitry, each single flex layer has to be covered individually and pressed. The coverlay foil is either used to cover the full area leading to a high material price, or used on selected areas, which incurs high manpower costs. In either case, the dry film or foil-type coverlay is connected to the laminate in a separate press step. The present invention relates to improvements to the covering layer on flexible circuit boards.
The requirements for developing a thermal curing FPC are many. A solder mask for use in a thermal curing FPC advantageously has some or all of the following properties:                Rheology and Thixotrophy resembling a typical screen printed soldermask—high low shear rate structure with the corresponding low high shear rate structure. See FIG. 1.        Processing with existing printing equipment (screen, spray, curtain coat, roller coat, pad print, gravure, flexo, offset, inkjet and spin coating) without any capital expenditure.        Screen open time minimum 4 hours.        Cures at max 150° C. (until lamination stage)        Must withstand lamination temperatures (typically 180° C. for 70 minutes or 220° C. for 40 minutes)        Double coating without adhesion problems, gitterschnit X-hatch of GT0-GT1        Final properties of PI foils need to be matched or exceeded        Dielectric Strength IPC TM650 2.5.6A minimum of 100 KV/mm        Isolation resistance IPC TM650 2.5.9L        Surface resistance IPC TM650 2.5.17L minimum of 1 megohm        Water uptake maximum of 1%        Solder bath resistance IPC TM650 2.4.13 (minimum 260° C. for 2 seconds)        Pass all other final finishes—Nickel-gold and chemical tin with no removal after tape testing.        Resolution of 50/50 μm tracks and gaps with a 25 μm film thickness on all substrates (PI, FR4 etc)        Minimum of 10 μm coverlay on top of 1 oz copper with a 25 μm coating        Flammability rating of UL 94V-0        Resistant to immersion in the following solutions (1 hour): IPA, MEK, Dichloromethane        Z-axis CTE—15 ppm below Tg, 80 ppm>Tg                    A=as supplied            L=after storage at 96 h, 20° C., 65% r.h.                        
U.S. Pat. No. 7,364,799 (Toyo Boseki) and WO 2008/072495 (Toyo Boseki) disclose polyamideimide (PAI) resins for application to a flexible medal-clad laminate.
WO 2008/041426 (Hitachi) (also published as EP 2 070 961) discloses a polyamideimide (PAI) resin for a flexible printed circuit board. The PAI has at least one terminal functional group selected from a carbonyl group, an amino group, an acid anhydride group and a mercapto group to enhance the heat resistance.
A problem exists in the use of polyamideimide (PAI) resins as coverlays in that they show poor viscosity stability, this is due to the fact that residual isocyanate groups remaining from the synthesis can react with pendent carboxylic acid groups causing the viscosity to rise with time. If the ratio of isocyanate is reduced to counter this in the synthesis of the PAI then the amount of amide groups are reduced with an increase in the level of imide functionality which results in the solubility of the polymer being much reduced.